Hemiparesis (loss of motor control and strength in the arm and leg on one side of the body) is the most common problem after stroke, the leading cause of adult disability. Most recovery of strength and control occurs in the first 3 months post stroke. Subsequent recovery is mainly the result of learning to compensate with residual capacities. Animal models suggest that the early recovery occurs as the result of interactions between up-regulated repair processes around the region of the stroke (heightened plasticity) and training regimens. There has been very little study of the early period of stroke recovery in humans. In this proposal we will use two principal approaches to determine what distinguishes the state of the brain in the first 3 months after stroke compared to the next 6 months. The first approach uses non-invasive brain stimulation methods to test the hypothesis that the human brain has heightened excitability and plasticity in the first 3 months after stroke compared both to later after stroke and to age-matched controls. The second approach tests the hypothesis that patients are more responsive to motor training in the first 3 months after stroke compared to subsequent months. That is to say, patients early after stroke may be particularly good learners. Conformation of our two hypotheses about a time window of heightened plasticity and increased responsiveness to training will have important implications for future innovations in neuro-rehabilitation. First, if it is possible to detect the state of heightened plasticity in any given atient then it could be used to devise an algorithm for how to plan rehabilitation. For example, if heightened plasticity were still present then rehabilitation of impairment would be emphasized rather than teaching compensatory strategies that rely on spared capacities. If plasticity were seen to have returned to normal levels then compensation would be emphasized. Second, having assays of heightened plasticity will allow assessment of pharmacological and other approaches that attempt to prolong, augment or re-open this critical time window of recovery.